The power supply device of the so-called switching mode comprises a switching element which is opened or closed in the state of high frequency so as to convert the alternating current input into the direct current output. In light of the advantage that the component parts of such a power supply device as described above can be miniaturized, the power supply device is widely used along with the electronic or communication equipment. However, the power supply device of the switching mode is by no means free from deficiencies. For example, the secondary side of the power supply device of the switching mode is vulnerable to leakage current due to a poor insulation or pressure resistance of the secondary side. If the leakage current is allowed to persist, the power supply device is prone to become a potential safety hazard. None of the conventional switching mode power supply device is provided with protection against leakage current.